1. Field of the Invention
The present invention relates to an electroless copper plating solution and a process for the formation of a copper film with this plating solution. More particularly, the present invention relates to an electroless copper plating solution for forming all copper films, such as copper films used for conductor circuits of printed circuit boards, copper films for conductor circuits on ceramic substrates, and copper films to be used for electromagnetic wave shielding materials, and a process for forming copper films by using this plating solution.
2. Description of the Related Art
As the electroless copper plating solution for electrolessly depositing metallic copper, there is widely known a solution comprising ethylenediaminetetraacetic acid (EDTA) or Rochelle salt as the complexing agent for a copper ion, and a solution comprising copper sulfate as the copper salt and formaldehyde as the reducing agent is most widely used. Research into complexing agents such as N,N,N',N'-tetrakis(2-hydroxypropyl)ethylenediamine and nitrilotriacetic acid has been made.
When these complexing agents are used, however, the electroless copper deposition speed is very low and usually 1 to 2 .mu.m/hr. Namely, since additives are incorporated to improve the physical properties of the obtained copper film, the deposition speed is reduced. In the basic plating solution free of additives (consisting solely of a copper salt, a complexing agent, a reducing agent and a pH-adjusting agent), the deposition speed is about 10 .mu.m/hr at highest. It was a recently reported that a plating solution giving a highest deposition speed is a solution comprising N,N,N',N'-tetrakis(2-hydroxypropyl)ethylenediamine as the complexing agent and an activator, and a deposition speed of 72 .mu.m/hr is obtained by this plating solution (Japanese Unexamined Patent Publication No. 59-25965). It has been also reported, however, that even if the above-mentioned plating solution is used, an applicable deposition speed is 2 to 5 .mu.m (Japanese Unexamined Patent Publication No. 60-15917).
Current demands for an electroless copper plating solution having a high deposition speed are increasing, to reduce the cost of, for example, printed circuit boards. To meet this demand, there have been proposed a plating solution comprising an accelerator (Japanese Unexamined Patent Publication No. 60-159173) and a plating solution formed by adding an activator to a reducing agent (Japanese Unexamined Patent Publication No. 55-76054). These plating solutions, however, are not satisfactory, and the development of a plating solution showing a higher plating speed is required.
The present inventors previously showed that, by using a monoamine type trialkanolamine, especially triethanolamine, as the complexing agent and making this complexing agent function also as an accelerator, electroless copper plating can be performed at a speed as high as 100 .mu.m/hr or more, and even if an additive is added to improve the physical properties, a copper film having good physical properties can be formed at a speed as high as 30 to 120 .mu.m/hr (see the specification of Japanese Patent Application No. 62-273493 published Jul. 4, 1989 as 1-168871).
Triethanolmonoamine acting as the complexing agent and accelerator in the above-mentioned high-speed electroless copper plating solution has a high stability in the form of a complex, and therefore, the reactivity is low and initiation of the reaction (plating) is not uniform. Accordingly, in the above-mentioned high-speed electroless copper plating solution, there is a need to easily initiate a stable plating reaction.
For example, if this high-speed electroless copper plating solution is applied to the full-additive preparation of a printed circuit board, then the formed copper film should have excellent physical properties.